In commonly owned patent application Ser. No. 383,175 U.S. Pat. No. 4,433,677 issued Feb. 28, 1984 a distraction splint is disclosed having two like anchor screws extending along respective longitudinal screw axes and adapted to be anchored in the pelvis with their axes crossing. Each of the screws has a head formed with a transversely open recess centered on a respective transverse axis and with an annular array of teeth centered on and angularly equispaced about the respective transverse axis. A bolt extends through one of the recesses and is threaded in the other recess. Thus the bolt secures the heads together with the transverse axes coaxial and the teeth interengaged. Respective distraction rods each have one end braced on the respective screw and another end braced oppositely against a vertebra.
This device is relatively effective, but can only be used to stabilized a straightened spinal region, not to straighten one. Thus a separate straightening device is usually employed to straighten out the spine, whereupon the splint is attached. The splint itself is not used for the straightening operation as the distraction rod is only limitedly longitudinally extensible.
Such a straightening device is described in commonly owned patent application Ser. No. 383,272 (U.S. Pat. No. 4,433,677 issued Feb. 28, 1984. This device serves to straighten a spinal column having a succession of vertebrae extending along a nonstraight line lying generally in a plane and has an elongated bar lying generally in the plane of the line and having a pair of relatively longitudinally displaceable bar parts in turn having respective bar ends. A pair of connectors at the bar ends are secured to respective vertebrae of the succession. Respective pivots between the connectors and the respective ends define therebetween respective generally parallel axes transverse to the plane. A main abutment movable along and fixable on one of the parts and longitudinally engageable with the other of the parts serves for limiting relative longitudinal displacement of the parts toward each other. Thus the main abutment can be moved along the one part to increase the spacing between the ends and thereby can straighten the succession of vertebrae between the connectors.
It is also known from German Pat. No. 2,649,042 based on an application filed Oct. 28, 1976 by M. B. Ulrich to provide an implantable splint that constitutes part of the device that actually does the straightening, and that can be left in the patient after the spine is straightened to keep the hitherto curved portion of the spine straight, although it is noted of course that the term "straight" is relative only as virtually all parts of the spine are at least gently curved. This splint has a threaded rod that is bendable and extends through a succession of anchors secured to the vertebrae to be straightened. Another implement is employed to initially straighten the succession of vertebrae, with concomitant adustment of pairs of nuts on the threaded rod that flank the respective anchors. Once the desired straightness is established, the pairs of nuts are tightened against the respective anchors to lock in the set position. This system allows the patient to be closed up after surgery, but still entails considerable work for the initial straightening operation. In addition the rod or rods, which must be flexible in order to follow the original curved line of the anchors, cannot be very rigid, so that occasionally the spine will at least partially revert to its initial nonstraight position.
Another principal disadvantage of all these systems is that they are only effective at exerting longitudinal tension. They do not exert any compression at all, and in fact can come unhooked relatively easily if, for instance, the spine is momentarily bent forward. In addition it is impossible to vary the length of the standard distraction rod once it has been installed. Length adjustment of any significant or useful amount is only possible by swapping the implanted rod with one of a different length, a surgical procedure that is obviously complex and normally considered excessive. Thus the known implants are not usable on a nonadult patient, as they cannot be adjusted as the patient grows.